Crystal growing procedure



` June 30, 1959 G. w. RusLER 2,892,739

' CRYSTAL GROWING PROCEDURE Filed oct. 1. 1954 INVENTOR ATTORNEY ,GEoRGEwl RusLvl-:R

BY'U? A! GAS OUTLET United States Patent O M' CRYSTAL GROWING PROCEDUREGeorge W. Rusler, Minneapolis, Minn., assignor to Minneapolis-HoneywellRegulator Company, Minneapolis, Minn., a corporation of DelawareApplication October 1, 1954, Serial No. 459,685

3 Claims. (Cl. 14S-1.5)

The present invention relates to a procedure and apparatus for growingcrystals having a substantially constant composition, and moreparticularly to growing of semi-conductor crystals having substantiallyconstant composition characteristics over an extended portion of theirlength.

According to procedures presently utilized in semiconductor crystalgrowing applications, a single-crystal ingot is grown from a melt, theusable portions of the ingot separated from the remainder of the ingotand the usable portion further processed. Generally, the usable Yportions of the ingot amount to only a relatively small portion of theentire ingot and for this reason the operation is considered ratherineflicient at this point.

The usable portions of the crystals may be increased somewhat if therate of pull is properly programmed. That is, the solid-liquidsegregation constant of the mixture may be increased or decreased inaccordance with the manner that the crystal is pulled from the melt, forexample, if the crystal is pulled mo-re slowly, the segregation constantdrops, and the concentration of impurities in the crystal as pulledremains fairly constant over a relatively longer portion of the crystal.This is due to the slow increase in impurity concentration as thecrystal pulling progresses. While this method is generally practiced, itis not entirely satisfactory since the size of crystals produced in thismanner is limited, and further, the programming rate requires a skilledoperators constant attention over long periods of time. It is Well knownthat in any system wherein the solid and liquid phases are inequilibrium with each other, portions of the impurities present tend tomigrate to one phase or the other, and in particular in present daysemi-conductor work, the impurities tend to migrate to the liquid phase.Therefore, as a crystal is being withdrawn from a melt the compositionof the liquid phase is constantly changing, that is, the liquid portionbecomes more heavily contaminated with impurities or in other wordscontains a relatively higher percentage of impurity members. Accordingto my improved procedure, the composition of the liquid phase is heldsubstantially constant by the addition of new raw material to the meltat a rate substantially equal to the rate of withdrawal of material inthe form of the crysalline ingot. Of course, the composition of thematerial added to the melt is substantially the same as the compositionof the withdrawn ingot. Accordingly, an ingot is obtained having acomposition which is relatively constant from one end to the other, anachievement which has been heretofore impossible to accomplish. In'otherwords, an extended portion of the crystal has a useful range ofcomposition. According to present day procedures, one would need anindefinitely long crystal' to obtain a substantial amount of usablematerial from a single crystalline ingot. In other words, my procedurenow makes it possible to obtain a singlecrystal ingot which has only asmall portion of waste material therein. Therefore, fewer crystalgrowing cycles are needed in order to provide the same quantity of p2,892,739 Patented June 30, 1959 ICC It is still a further object of thepresent invention 'to provide apparatus for growing the improvedcrystals as set forth herein.

The invention may be more easily and fully comprehended with referenceto the accompanying drawing in which:

The figure is a vertical sectional view of a crystal growing apparatusparticularly adapted for carrying out the present invention.

The improved process of the present invention is conveniently carriedout in the crystal growing apparatus as illustrated in the accompanyingdrawing. Accordingly, there is provided a crystal growing assemblygenerally designated 10 which includes a crucible system 11 and crystalpulling mechanism generally designated 12 mounted within the shell orhousing 14. Metal feeding means 15, heating coils 16, and inert gassupply and exhaust tubes 17 and 18 respectively are also included in thesystem, and contained at least partially within the housing 14. Thecrucible system generally designated 11 includes an inner container 20surrounded and spaced from an outer shell 21. The space between theinner and outer shells 20 and 21 defines an annular chamber as at 22.Spaced ribs 24 are provided in the annular chamber 22 in order to holdthe inner and outer shells 20 and 21 respectively in relatively spacedrelationship. The inner container 20 is provided with a hole or port at25 which provides communication between the inner chamber 26 and theouter annular chamber 22. The crucible system 11 is mounted on the plate28 which is situated on the shaft 29 and adapted for axial rotationtherewith. Shaft 29 is adapted for axial rotation in the bearing 29A.

The crystal pulling mechanism 12, includes a pulling shaft 30 which isprovided with a seed crystal retaining member 31 adapted to retain aseed crystal 32 by any convenient means, such as, for example, the setscrew 33. In operation, the ingot 35 is fused onto the seed crystal 32and an extended ingot is formed or grown as the crystal pullingmechanism 12 is moved in the direction of the arrow 36 at a proper ratefor growing or forming of the ingot 35.

The crucible 11 is heated by the induction heating coils 16, which aresupplied with high frequency energy from an external source ofconventional or well known design, not Sho-wn. The coils 16 are providedwith cores 37 through which a suitable coolant, such as water, may ilow.

The metal feeding means 15 includes a hopper member 40 containing aquantity of finely divided semi-conductor metal as shown at 41. Controlmeans, such as the damper 42, are provided in the shaft 43 which extendsfrom the hopper 40 to a point directly above the annular melting chamber22. Control means 42 are adapted to permit passage of solid materialinto the chamber 22 at a rate substantially equal to the rate at whichmaterial is drawn from the system in the form of the crystalline ingot35, thereby maintaining the quantity of metal within the crucible system11 at a constant level at all times, even when the crystalline ingot isbeing pulled.

The improved process of the present invention may be convenientlycarried out inthe apparatus described hereinabove as follows. Inoperation, the crucible is filled with a charge of doped germaniumhaving a resistivity which is lower than that desired in the growncrystalline ingot product'. Since in germanium the ratio betweenimpurity content in the solid phase to thatin the melt, the segregationconstant, is very low, a charge having a bulk resistivity substantiallylower than that desired in the ingot is rutilized. g In other words, theimpurity content of the charge is higher than that desired in the ingot.The charge is heated to a surface temperature of 940 F. and the crucibleis then set into rotation about its axis, along with. the shaft 29; Atthis time, the seed crystal 32 is lowered and placed in contact with thesurface of the melt, permitted to' remain there for about 30 seconds, oruntil the crystal commences to form about the seed, at which time theseed crystal is slowly withdrawn, at a rate such that the crystallineingot 35 forms thereon. The shaft 30 is drawn upwardly at a relativelyconstant rate, that is, at a rate substantially equal to the rate offorming of the crystalline ingot. For germanium, this rate is about 25mils per minute at a surface temperature of 940 F. and withV thecrucible rotating at about 150 r.p.m. When the crystalline ingot beingwithdrawn from the melting zone 26- reaches an optimum composition asindicated by its-resistivity, new material having a resistivity orimpurity content substantially equal to that of the withdrawn ingot 35is added to the melting zone 22 through the conduit or shaft 43 whichextends from the hopper 40. The proper times for adding new material tothe melt is readily determined by practice. This new material ispermitted to melt under the influence of the induction heating coils 16,and under static pressure influence, eventually passes through the port25 into the molten zone 26 of the inner crucible 20.

It is preferable that the new material added to the melting zone 22 bepermitted to reach a. suiiiciently high temperature for the period oftime necessary for the material to become thoroughly molten and therebylose its memory of crystallization, before it reaches the port 25. Inpractice, it may be necessary in some instances to commence the pullingof a crystal before additional new material is added to the melting zone22. This would ocour in instances Where the crystal ingot being pulledhas an original resistivity which is higher than that desired in thefinal product. It is important that the addition of new material ismaintained substantially at the rate of withdrawal of material in theform of the crystalline ingot, the volume of metal in the crucibleremaining substantially constant. This insures that the resistivity ofthe crystalline ingot as it is withdrawn will likewise remain constant.

Example In order to prepare al crystalline ingot having a desired lresistivity of from, for example, 4 to 6 ohm-centimeters, the Crucible11 is filled with a charge of n-type antimony doped germanium having abulk resistivity of about 300. times that desired in the crystal. Ofcourse, other doping substances may be utilized, such as arsenic,phosphorous, bismuth, indium or the like. The charge is then heated to asurface temperature of 940 F. and the seed crystal immersed a distanceof 1/32 inch into the surface of the melt contained in the molten zone26 of the crucible 11. The seed crystal is permitted to remain incontact with the surface of the melt for about 30 seconds beforerotation of the crucible is commenced, rotation is begun and a slow,constant withdrawal ofthe ingot is then started. For a' temperature of940 F., the ingot is withdrawn from the Crucible at a rate of about 11/2inches per hour, and an ingot having a diameter of between l and 2inches is formed. The segregation constant for this material is 0.003for a non-agitated solution and 0.005 for a solution agitated and acrystal pulled at the rate set forth in this example. Upon commencementof withdrawal of the ingot from the melt, control member 42 is openedand additional material 41 comprising bulk germanium n-type, doped withantimony to a resistivity of from 4 to 6 ohm-centimeters is permitted toenter the system by way of the melting zone 22. This addition ofmaterial is closely controlled at a rate substantially equal to the rateof withdrawal of material from the crucible system in the form of aningot, and the bulk composition of this substance is substantially thesame as the composition of the ingot.

Although specific reference has been made to germanium in thisapparatus, the method is equally applicable to silicon and othermetallic systems. As a slightly modified procedure in accordance withthe present invention, it is possible to obtain an elongated crystalhaving a composition which varies over a desired impurity range byvarying the rate of addition or composition of the addition material. Inthis regard new material having a composition substantially equal tothat desiredy in the final crystal may be added to the melt as the ingotis Withdrawn. Of course, as the drawing of the ingot is continued,- thecomposition of the ingot will approach that of the added material, andclose control of product composition is therefore possible. Thiscomposition control may also be achieved by the rate at which newmaterial is added to the melt, since generally the composition of themelt will be more heavily contaminated than that of the ingot drawntherefrom. Therefore the ingot composition will approach a more heavilycontaminated level as the quantity of the melt decreases due to lower orno addition of new material to the melt. This feature may be utilizedfor varying the composition of the ingot by a controlled addition of newmaterial at a rate which Varies from the rate of withdrawal.

Although various specific embodiments of the invention herein have beendisclosed, it will be understood that there is no intention to limit thescope of the present invention to these specific embodiments alone,since they are used for purposes of illustration only. Many details ofcomposition and procedure may be varied Without departing from theprinciples of this invention. It is therefore not my purpose to limitthe patent granted on this application otherwise than necessitated bythe scope of the appended claims.

I claim as my invention:-

l. The method of growing a uniformly oriented body of a semiconductormaterial selected from the class consisting of germanium and silicon andincluding a substantial portion with a uniform and predeterminedconcentration of a certain doping impurity dispersed therethrough from amelt of said semiconductor material including a concentration of saiddoping impurity which is greater than said predetermined concentration,said method comprising withdrawing saidl body from said melt at apredetermined rate, and maintaining said melt at constant volume andconstant concentration of said doping impurity after said body hasacquired said predeterminedl concentration at the interface with saidmelt by simultaneonsly adding new material to said melt at a rate equalto the rate at which the material is being Withdrawn from said melt inthe formation of said body and melting said new material in said melt,said new material consisting essentially of said semiconductor materialand said dopingA irnpurity, with the concentration of said dopingimpurity in said new materiall being equal to said predeterminedlconcentration.

2. The method'of growing crystalline ingots of a serniconductor materialselectedffromA the class consisting of germanium and silicon and havinga controlled rst predetermined concentration of a certain dopingimpurity which method includes providing a melt of said semiconductormaterial together with a second predetermined concentration of saidcertain doping impurity, said second predetermined concentration of saidcertain doping impurity being greater than said controlled firstpredetermined concentration thereof, and withdrawing an ingot from saidmelt at a predetermined rate, a substantial portion of which ingotcontains said certain doping impurity in said controlled firstpredetermined concentration while maintaining the volume of the melt andthe concentration of said certain doping impurity therein constant aftersaid body has acquired said predetermined concentra- "tion at theinterface with said melt by simultaneously adding to the melt additionalamounts of said semiconductor material and said certain doping impurityat a rate equal to the rate at which the material is being Withdrawnfrom the melt in the formation of said ingot and melting said additionalsemiconductor material, the concentration of said certain dopingimpurity in the newly 15 2,739,088

References Cited in the tile of this patent UNITED STATES PATENTS1,353,571 Dreibrodt Sept. 21, 1920 2,553,921 Jordan May 22, 19512,651,831 Bond et al. Sept. 15, 1953 2,709,842 Findlay June 7, 19552,727,839 Sparks Dec. 20, 1955 Pfann Mar. 20, 1956

